I gave it some thought. I’ve distilled the reddit value proposition for me at least. Imagine having to manage a separate login for a different forum for every topic you’re interested in. And having to check 10+ forums a day for responses to your comments. With reddit it’s a common interface to every topic I’m interested in. I’m subscribed to about 60 subreddits now, from off grid cabins to deep learning to Spacex.
Bookmarks folder + Browser password manager + email notifications solves this for N forums where N is less than ~20.
Sure, it's harder to have a really shallow level of engagement in a community if it's not all algorithmicly curated on one page (you actually have to go to the forums and check what's up vs seeing just the popular stuff in front of you) but I think for most interests that's probably a good thing for the community around that interest.